Heretofore, cold rolled steel sheets have been mainly used, for example, for automobile outer panels. However, in accordance with the requirements for weight reduction of automobile bodies, the use of aluminum alloy sheets such as an Al—Mg-based alloy sheet and an Al—Mg—Si-based alloy sheet has been studied recently. Particularly, the Al—Mg-based alloy sheet has been proposed as a body sheet for motor vehicles because it is excellent in strength, formability and corrosion resistance.
Heretofore, as a process for producing such an aluminum alloy sheet, there has been employed a process including casting a slab by DC casting, face milling both surfaces of the slab, homogenizing the face-milled slab in a soaking furnace, and subjecting the homogenized face-milled slab to hot rolling, cold rolling, intermediate annealing, cold rolling, and final annealing to finish it to a predetermined sheet thickness (refer to Patent Document 1).
On the other hand, there has been proposed a process including continuously casting a thin slab with a belt caster, directly winding the resulting thin slab into a coil, subjecting the coiled thin slab to cold rolling and final annealing to finish it to a predetermined sheet thickness. For example, there is disclosed a process for producing an aluminum alloy sheet for motor vehicles excellent in press formability and stress corrosion cracking resistance (Patent Document 2). This process comprises preparing a melt comprising 3.3-3.5 wt. % Mg and 0.1-0.2 wt. % Mn and further comprising at least one of 0.3 wt. % or less Fe and 0.15 wt. % or less Si, a balance being ordinary impurities and Al; casting the melt into a thin slab having a thickness of 5 to 10 mm in a twin-belt caster at a speed of 5 to 15 m/min so that the cooling rate at ¼ depth of the thickness of the thin slab is 40 to 90° C./sec; winding the resulting thin slab into a roll; cold rolling the rolled thin slab with a roll having a surface roughness of 0.2 to 0.7 μm Ra; and annealing the cold rolled thin sheet.
However, in the above process, since 0.1-0.2 wt. % Mn is contained in the chemical composition of the melt for the purpose of refining the recrystallized grains and the solidification cooling rate is relatively fast, the size of intermetallic compounds such as Al—(Fe.Mn)—Si is reduced to resulting in excellent formability. On the other hand, there is a problem that, since the amount of dissolved Mn in the matrix is excessively high, yield strength is higher and spring back after forming is increased.
In order to solve this problem, for example, a so-called stabilization treatment is proposed (Patent Document 3) in which a continuously cast and rolled sheet of an aluminum alloy containing 3-6 wt. % Mg is subjected to annealing treatment followed by straightening, heated at a predetermined temperature of 240 to 340° C. for 1 hour or more, and then slowly cooled.    Patent Document 1: Japanese Patent No. 3155678    Patent Document 2: International Publication No. WO 2006/011242    Patent Document 3: Japanese Patent Laid-Open No. 11-80913